In permanent magnet motors, interfering or parasitic torques may develop because of the interaction between the edges of the magnetized poles and the slots of the stator. In small size motors, such as are used in disc drives, these problems can be significant. The radial thinness of the rotor magnet is required by the limited diameter of the motor itself. In such a small, high power motor, the air gap between the facing surfaces of the stator and rotor must also be minimized. The combination of high induction, the slotted stator, the minimal air gap, and the thinness of all the elements can easily lead to so-called torque unevenness or torque ripple.
The resulting running torque ripple in the motor is a source of audible noise. Since the disc drive is used in computers, such audible noise must be reduced in order for the product to be commercially successful.
It has been recognized that the reduction of torque ripple is an important goal. One current method known of reducing torque ripple is designing the spindle motor to have a trapezoidal back EMF (BEMF) wave form. Current methods used to achieve such trapezoidal BEMF include weak magnetization of the magnet pole center, or selection of stator slot/number/magnetic pole member. For example, it is disclosed in European patent 291,219, U.S. Pat. No. 4,847,712 Issued Jul. 11, 1989, that the number of stator poles should be kept approximately the same as the number of rotor poles. This is also taught to reduce torque fluctuations during starting or idling. However, in general according to the reference, this method requires at least nine wound coils per motor.
A problem with this approach is that in most known spindle motor configurations, other design considerations dictate the number of poles and slots to be used and the numbers are usually different. The selection of the number and arrangement of the poles and slots in most disc drive spindle motors is dictated by other considerations than the shape of the back EMF wave form.
Another approach has been to reduce the magnetic skew angle between adjacent poles. Reducing skew angle does reduce acoustic prominent tone amplitudes. However, this also causes a significant increase in torque ripple and cogging torque.
Thus, the problem remains of providing a magnet for use in a motor which minimizes the motor's running torque ripple while remaining consistent with other design considerations for an efficiently operating disc drive spindle motor.